I will sum up in a nutshell. I'm using a medium yarn as double strand with size 15 needles. I need a short gauge sample (enough for 4 inches) smiley. I got 9 stitches per 4 inch. I'm making a washcloth that is 8.5 in width (measured a home cloth). How do I determine how much to cast on? When I do per inch I get 2.5 then times by 8.5 and get 21.25. Calculating with the 4 inch leaves me with 16? What am I doing wrong? Help and THANK YOU.

9 sts /4" is 2¼ sts per inch, not 2½. Then 2.25 times 8.5 is 19.125, so you would need 19 or 20 sts depending on if you need an odd or even number of stitches. The 4" measurment is just used to determine the stitches per inch, and doesn't come into the calculations again - you just need the sts per inch times the width of what you want.